Most buildings require a flat foundation formed by pouring concrete into a form on the ground. However, after concrete hardens it is porous and therefore allows moisture, soil gases (i.e. Radon), alkaline salts, and soil sulfates to travel from the earth through the pores into either a building or other structure that has been formed on top of the concrete slab. Moisture is a broad term and includes, without limitation, water vapor, liquid water, and any and all forms and phases of water. This is a major cause of building defects and contributes to serious problems with the concrete foundation, floor coverings, and indoor air quality.
To prevent moisture and other permeating substances from migrating through the concrete slab, a geo-membrane is commonly placed on the ground before the concrete foundation is poured onto the slab. A geo-membrane generally retards or nearly completely prevents moisture and other permeating substances from rising out of the soil and permeating into the concrete slab above by preventing its migration through the barrier. This is only effective, however, if the geo-membrane below is relatively free of any open seams between sheets of the geo-membrane or substantial gaps around the perimeter for the permeating substances to leak through into the concrete slab.
Generally, to install a geo-membrane, it is laid down on the ground in sheets that must be connected in order to form one continuous barrier below the concrete slab. Seaming tape with a single side of adhesive is the usual way of sealing the sections of the geo-membrane together to provide one continuous seal below the concrete slab. This method is only effective for keeping the partitions together if the soil or void forms beneath the geo-membrane do not settle or shift unevenly.
Frequently, however, certain types of soil will sink or settle beneath the foundation after it has been poured, especially if the slab is supported by concrete piers. This causes the geo-membrane to sag beneath the foundation which, in turn, causes the seams between the section of the geo-membrane to separate. This comprises the complete permeating substance seal below the foundation and exposes the building structure to a plethora of damage from permeating substances rising from below the concrete foundation, including mold, air pollutants and other consequences of moisture and soil gas infiltration.
Lastly, after the foundation is poured over the geo-membrane and is in place a different problem arises. Generally, a concrete foundation is cut at various positions to induce cracking and separation during the shrinking of the concrete that takes place during setting. This forms gaps between various portions of the concrete which accommodates expansion and contraction through the seasons and prevents damage to the slab during such expansions and contractions. Unfortunately, however, the geo-membranes tend to have such low surface friction that as the concrete shrinks, it encounters less resistance to sliding across the geo-membrane. Therefore, instead of cracking and separating at all pre sawed locations in the slab, the concrete separates at only a select few dominant control joints. With such low surface friction on the concrete slabs, there is not enough force to separate all of the joints. Instead, most of the control joints remain fused and slide along the geo-membrane with the rest of the concrete. This produces an unpleasant look with large gaps in a few locations of the slab and may promote other problems resulting from the change structural and mechanical properties of the slab as a whole.